Planters of the type having a main frame with a plurality of individual planting units mounted along the frame and adjustable to vary row width are well known. Many of the planter configurations include a sprocket and chain drive assembly, or transmission, permanently mounted between the individual planting units, generally in the middle and towards the rear of the main frame. A drive or ground wheel assembly mounted on the frame provides drive to the transmission which in turn rotates a drill shaft connected to the individual planting unit seed meters. By driving a plurality of planting units from a transmission, the need for a separate drive wheel for each individual unit is eliminated, and the seeding rate can be quickly and easily adjusted at the transmission. However, when relatively narrow row spacings are desired, the transmission has been found to interfere with the adjacent planting units. In many cases the desired narrow row spacing cannot be obtained because of the transmission. It has been necessary to use a different planter for narrow row crops than for wide row crops, or alternatively to use a wider spacing between the planting units on either side of the transmission than that used between the rest of the planting units. The latter method leaves a gap which is objectionable or unacceptable to the farmer. An alternate transmission and drive assembly is required to accommodate the narrow row spacing capabilities without the need for a separate drive wheel for each unit.
Another problem encountered with the permanently mounted center transmission is the lack of versatility in spacing the individual planting units on the frame. Often the location of the transmission interferes with the desired location for a particular planter unit, especially when an odd number of units are to be used and symmetrically located with respect to the center of the frame or when planting units are to be spaced between units already mounted on the frame.